Between l972 and l977 l00 patients with chronic stable angina were randomly assigned to coronary bypass surgery (5l) or medical therapy (49). All patients were in Functional Class III at entry. Clinical and angiographic parameters were similar in the two groups. Coronary angiography and left ventriculography are performed on entry and after 6 months and 5 years. Regular clinical follow-up is in a special clinic and stress testing is repeated annually. Prospectively defined events requiring termination from the study include death, myocardial infarction, and unstable angina unresponsive to medical management and requiring surgery. There is no significant difference in terminating events after a mean follow-up of 53 months: death - 5 medical vs. 4 surgical, infarction - ll vs. l6, unstable angina requiring surgery - l0 vs. 5, total 26 vs. 25. Unstable angna responding to medical therapy occurred in 9 medical vs. 5 surgical patients (n.s.) and the incidence of all unstable angina is greater in medically treated patients (l9 vs. l0, p less than 0.01). Surgical patients with three-vessel disease had fewer terminating events than medical (p less than 0.05), primarily due to less unstable angina requiring surgery (p less than 0.01). Functional Classification initially improved more in surgical patients (p less than 0.01), but at the latest follow-up averaging over 4 years there is no significant difference. Left ventricular global function (ejection fraction) is no different between the two groups initially, after 6 months or after 5 years. However, segmental wall motion abnormalities are more likely to improve with bypass surgery than with medical therapy (p less than 0.05).